Processes for the formation and development of images on the imaging surfaces of photoconductive materials are well documented. These processes have in common the steps of employing a photoconductive insulating element which responds to imagewise exposure with actinic radiation (radiation which is absorbed by and effects a chemical or electrical change in a material) by forming a latent electrostatic charge image. The image is then made visible by electrically charged toner particles which are attracted to it. These toner particles are carried in a developer, which can contain either a dry or a liquid carrier. The visible image can then be transferred to a receiving sheet such as paper or it can be fused or otherwise fixed directly to the photoconductive element.
Various types of organic and inorganic photoconductive insulating elements are known for use in electrophotographic imaging processes. One particular type of photoconductive element is photoconductive recording film which comprises an organic or inorganic photoconductive compound dispersed in a polymeric vehicle to form a heterogeneous or homogeneous photoconductive insulating layer which is carried on a flexible polymeric film support. The resulting photoconductive film is stored and used in roll form. When photoconductive recording film is used, it is first sensitized by uniformly charging the surface with a corona charger. Then it is exposed to an appropriate light source and developed, for example, with a liquid developer. The image is then dried and fused directly to the surface of the photoconductive recording film.
When the photoconductive recording film is new, the resulting developed image is of good quality. It has been noticed, however, that film that has been stored in roll form for a period of time produces images with imperfections. Specifically, these imperfections consist of random parallel lines that run perpendicular to the length of the roll film. The cause of these imperfections is not readily apparent. Considerable research has been carried out to discover the causes of these defects.
After extensive testing of different hypotheses, it has been found that the combination of storage at elevated temperatures and the use of liquid developer causes the described defects.
It is now believed that elevated temperatures cause the film to creep slightly during storage. Because the film is rolled on a spool, it tends to take on a curled configuration. This phenomenon is familiar to anyone who has tried to hang wallpaper. As one attempts to unroll the paper and apply it to the wall, the wallpaper stubbornly resists, preferring instead to remain in its rolled form. Roll photoconductive film behaves in an analogous manner. This inherent tendency of roll film to acquire a "set" on aging is referred to in the art as "core-set" when the set conforms to a core or spool on which the film has been wound and stored. "Core-set" may be explained as the result of plastic flow deformation of the film when it is wound on a core and stored at ambient temperature and humidity for a period of time sufficient for the film to acquire a substantially permanent curvature in the direction of its winding on the core. Although the core-set curling tendency of the polymeric film base itself is a significant force in causing curl, and can be the controlling factor in curling tendencies of a photoconductive film roll, the presence of coated layers on the base can also cause problems. This is because each layer of the film element differs from the others in its dimensional response to moisture and heat. The varying stresses of the different layers will, therefore, tend to cause the film element to assume a flat or curved shape governed by the equilibrium of the force exerted by the layers. These stresses, and the resulting amount of curl, will change as the temperature and humidity change. Core-set increases with an increase in storage temperature.
It has also been shown that the orientation of the coated photoconductive insulating layer affects the film's propensity to crack. When the insulating layer is wound on the inward side of the film element (i.e. the side of the film that faces the core of the film spool) the film exhibits more cracking defects. The hypothesis is that during storage, free volume in the insulating layer decreases and embrittlement begins. After the film has been stored for a time at elevated temperature, it takes on a core-set and thus when it is processed the equipment subjects it to stresses that induce minute cracks in the film which are not visible to the eye. These cracks might be of little consequence at this point. But, as has now been discovered, when a liquid developer contacts the cracks, they enlarge, growing in both length and width. The liquid developer becomes entrapped in the cracks and shows up on the final image as a line.
After discovery of the cause of the defects the difficult task remaining has been to eliminate the cracking defects without affecting the other desirable qualities of the film. This had to be accomplished in a way that was both economical and easy to implement.